1. Field of the Invention
This invention relates to a disc drive device and, for example, to the disc drive device of a still video camera which is arranged to perform frame recording with a field head.
2. Description of the Related Art
In performing frame recording with a field head, it has been practiced, for example, to move the field head from one track to an adjacent track by means of an ordinary head advancing mechanism.
As regards a head moving arrangement for this purpose, a discrete head moving mechanism is sometimes arranged for fine tracking adjustment or for high-speed head advancing in addition to the ordinary head advancing mechanism. However, this arrangement is not aimed at an improvement in precision of a track pitch between adjacent tracks.
The still video camera uses a still video floppy disc as a recording medium. According to the standards, each track of the still video floppy disc is allowed to have a radius tolerance within a limit of 14 .mu.m. Therefore, it must be taken into consideration that, in the worst case, a pitch error between adjacent tracks is 28 .mu.m. If frame reproduction is performed by using a frame head under such a condition, it is hardly possible to obtain any adequate reproduction output from two adjacent frame-recorded tracks with the frame head set in the same head position, because there is more than 20 .mu.m discrepancy in a track pitch between the adjacent tracks. Then, it is inevitable that the picture quality of the reproduced image degrades.
This can be prevented by increasing the degree of precision in pitch between the adjacent tracks at the time of recording. However, in order to increase the precision in pitch between any specific adjacent tracks, the pitch precision must be enhanced for every pair of adjacent tracks. In other words, it is necessary to control and ensure pitch precision of each of 49 track pitches in addition to the control over absolute track precision.
Further, in a case where a head advancing mechanism is arranged to advance the head by only one track at a time and the head is advanced over all the tracks by this mechanism, errors are accumulated every time the head is moved. Therefore, in order to meet the requirement for absolute track precision, the pitch precision must be guaranteed in the order of sub-microns.
To meet both the requirements for the absolute track precision and the pitch precision between adjacent tracks, it is necessary to have more controlled management for precision. Then, it incurs difficulties in terms of manufacture and usage.
In respect of the track advancing control method for accurately positioning the head at each of the tracks formed on a disc-shaped recording medium, the conventional still video camera system has been arranged as follows: a frame-recording image is recorded by recording two field images separately in adjacent tracks. In reproducing the frame-recording image by means of a field head, signals are read by moving the head to the two adjacent tracks one after another. In order to position the head for each of the tracks, the so-called ATF (automatic track finding) control is performed. Under the ATF control, a reproducing head is positioned in such a point at which the output of the reproducing head reaches its maximum value. An advantage of this method lies in that, a stable, high reproduction output is obtainable even if the degree of advancing precision of the head advancing mechanism is relatively low.
FIG. 4 is a graph for explaining the operation of the ATF control. The axis of abscissa shows the head advancing position. The axis of ordinate shows the output of the head. The head is assumed to be moved from one track to another track. Then, assuming that the head is first moved to a position A which is close to the desired track (a maximum output position), for example, the output of the head obtained at the position A is stored. After that, the head is moved to a very small extent (fine advancing), and an output thus obtained is compared with the previously stored output. This action is repeated for different head positions, in the sequence of positions A - B C - D - E, until the output of the head obtained at the current position becomes smaller than the output obtained at the immediately preceding position. Then, the output obtained at the immediately preceding position is subjected to a reproduction process. In obtaining the head output by the fine advancing of the head, at least a period of time corresponding to one rotation of the disc, i.e., 1 V (V: a vertical scanning period) is necessary. Since the period of 1 V is necessary in reading a recorded signal, a total period of 8 V is necessary for obtaining a reproduction output in the case of FIG. 4.
FIG. 5 shows a case where the head happens to be brought by the first step of the head advancing action to a point at which the output of the head becomes a maximum value. A total period of 6 V is necessary even in this case, because the head must be still moved in the sequence of positions A - B - C - D.
In a case where a frame-recording image is to be reproduced with a field head, the above-stated head advancing and positioning mechanism requires an excessively long period of time for obtaining the reproduced image, because the ATF control action must be performed twice.